Try to delete the pluginreg.dat file in the Firefox Profile Folder and maybe addons.sqlite as well to reset the plugin registry databases.
You can also try to delete the addons.sqlite file.
You can use this button to go to the Firefox profile folder:
*Help > Troubleshooting Information > Profile Directory: Show Folder
*http://kb.mozillazine.org/Profile_folder_-_Firefox

Question owner

Thank you. I tried that and still have the same duplicate entries.
Also, I have Flashblock and Noscript installed, but neither of them show up in the list.
Can you suggest any other possible solution?
Thanks.

Thank you. I tried that and still have the same duplicate entries.
Also, I have Flashblock and Noscript installed, but neither of them show up in the list.
Can you suggest any other possible solution?
Thanks.

It is best not to leave that pref set to true as it exposes that full path to web servers via the navigator.plugins object, so reset that pref to false after you are done with the about:plugins page.

You can also see that path in the pluginreg.dat file.

You can see from the description that the plugin have different purposes, so although they have the same version number, they are different.
You can set the plugin.expose_full_path pref to true on the about:config page to see the full path of plugins on the about:plugins page.
*http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins
It is best not to leave that pref set to true as it exposes that full path to web servers via the navigator.plugins object, so reset that pref to false after you are done with the about:plugins page.
You can also see that path in the pluginreg.dat file.

Question owner

I was having another issue where I could not see anything in the list of Extensions, although I knew there were many.
I unlinked this device from sync and did a reset, then re-synced.
Things seem to be working now.

I was having another issue where I could not see anything in the list of Extensions, although I knew there were many.
I unlinked this device from sync and did a reset, then re-synced.
Things seem to be working now.